1. Field of the Disclosure
The present disclosure relates to an organic light emitting display (OLED) device, and more particularly, to a light-emitting diode and a deposition apparatus for fabricating the same.
2. Discussion of the Related Art
Recently, a slim, light-weight, flat panel display having low power consumption has been developed and applied to various technical fields.
In an organic light-emitting display (OLED) device, charges are injected into a light-emitting layer formed between a cathode electrode which is an electron injection electrode, and an anode electrode which is a hole injection electrode to form electron-hole pairs, and light is emitted when the electron-hole pairs disappear. The OLED device, which can be formed on a flexible substrate such as a plastic substrate, provides excellent colors because it is a self-emitting type device, and also has low power consumption since it can be driven at a low voltage (below 10 V).
Since the OLED device, unlike a liquid crystal display (LCD) or a plasma display panel (PDP) device, can be manufactured by a very simple manufacturing process, a deposition apparatus and an encapsulation apparatus are the only manufacturing apparatuses required for manufacturing the OLED device.
Specifically, in an active matrix type, since the voltage for controlling current that is applied to pixels is charged in a storage capacitor to maintain a constant voltage until a next frame signal is applied, a light-emitted state is maintained while a screen is displayed, regardless of the number of gate lines. This operation is described with reference to FIG. 1, below.
FIG. 1 is a circuit diagram showing a sub-pixel area SP of a conventional OLED device.
As shown in FIG. 1, in the OLED device, gate lines GL, data lines DL, and power lines PL are arranged in a manner to cross each other, thereby defining a plurality of sub-pixel areas SPs, and in each sub-pixel area SP, a switching thin film transistor Ts, a driving thin film transistor Td, a storage capacitor Cst, and a light-emitting diode Del are formed.
The switching thin film transistor Ts is connected to a gate line GL and a data line DL, the driving thin film transistor Td and the storage capacitor Cst are connected between the switching thin film transistor Ts and a power line PL, and the light-emitting diode Del is connected to the driving thin film transistor Td.
When the OLED device displays an image, the switching thin film transistor Ts is turned on according to a gate signal applied to the gate line GL, and a data signal applied to the data line DL is applied to the gate electrode of the driving thin film transistor Td and one electrode of the storage capacitor Cst through the switching thin film transistor Ts.
The driving thin film transistor Td is turned on according to the data signal, and as a result, a current proportional to the data signal flows from the power line PL to the light-emitting diode Del through the driving thin film transistor Td, so that the light-emitting diode Del emits light with luminescence proportional to the current flowing through the driving thin film transistor Td.
At this time, the storage capacitor Cst is charged with a voltage proportional to the data signal to maintain the voltage of the gate electrode of the driving thin film transistor Td constant during one frame.
Accordingly, the OLED device can display a desired image using a gate signal and a data signal.
The light-emitting diode Del includes a first electrode connected to the driving thin film transistor Td, a second electrode that is opposite to the first electrode, and an organic light-emitting layer positioned between the first and second electrodes. For example, the first electrode may be an anode electrode, and the second electrode may be a cathode electrode.
The OLED device may be classified into a red light-emitting diode, a green light-emitting diode, and a blue light-emitting diode according to light-emitting patterns of red, green, and blue colors.
A light-emitting material for a general light-emitting diode may be made of a host and a dopant.
A general light-emitting diode is described with reference to FIG. 2, below. FIG. 2 is a cross-section view of a general red light-emitting diode RLE.
As shown in FIG. 2, the red light-emitting diode RLE includes an organic light-emitting layer positioned between an anode electrode 1 and a cathode electrode 7. The organic light-emitting layer includes an emitting material layer (EML) 4 disposed between the anode electrode 1 and the cathode electrode 7, a hole transporting layer (HTL) 3 disposed between the anode electrode 1 and the emitting material layer 4 to inject holes from the anode electrode 1 and electrons from the cathode electrode 7 into the emitting material layer 4, and an electron transporting layer (ETL) 5 disposed between the cathode electrode 7 and the emitting material layer 4. Also, in order to efficiently inject holes and electrons, a hole injecting layer (HIL) 2 is disposed between the anode electrode 1 and the hole transporting layer 3, and an electron injecting layer (EIL) 6 is disposed between the electron transporting layer 5 and the cathode electrode 7.
The emitting material layer (EML) 4 is formed of a host ho and a dopant do.
Lately, in order to obtain high luminous efficiency with a low voltage, a short-wavelength light-emitting material is used as the dopant do.
The short-wavelength light-emitting material means a light-emitting material in which a wavelength of the peak value of its electro luminescence (EL) spectrum is longer than a wavelength of the peak value of its photo luminescence (PL) spectrum. The EL spectrum represents intensity with respect to wavelengths of light emitted from a light-emitting material that emits light by electricity, for example, the EL spectrum represents intensity with respect to wavelengths of light emitted when electricity is applied between the anode electrode 1 and the cathode electrode 7. The PL spectrum represents intensity with respect to wavelengths of light emitted from a light-emitting material that emits light by light stimulus from the outside.
However, when such a short-wavelength light-emitting material is used as the dopant do, the following problems are generated.
The problems are described with reference to FIGS. 3A, 3B, and 3C, below. FIGS. 3A, 3B, and 3C are simulation results showing problems when a red short-wavelength light-emitting material is used as a dopant, wherein FIG. 3A is a graph showing changes in luminance with respect to viewing angles of light-emitting materials corresponding to red, green, blue, and white colors, respectively, FIG. 3B is a graph showing color changes with respect to viewing angles of the light-emitting materials corresponding to red, green, blue, and white colors, respectively, and FIG. 3C is a graph showing changes in luminance with respect to viewing angles of white color.
First, changes in luminance with respect to viewing angles are described.
As shown in FIG. 3A, in the cases of green (G), blue (B), and white (W) light-emitting materials, as a viewing angle increases, luminance decreases gradually. However, in the case of a short-wavelength red (R) light-emitting material, luminance increases according to an increase of a viewing angle until the viewing angle reaches about 40°, and after the viewing angle exceeds 40°, luminance decreases according to an increase of the viewing angle. In other words, the short-wavelength red (R) light-emitting material shows different changes in luminance with respect to viewing angles from the green (G), blue (B), and white (W) light-emitting materials.
Now, color changes Δu′v′ with respect to viewing angles are described with reference to FIG. 3B.
As shown in FIG. 3B, likewise, the red (R) light-emitting material shows different color changes with respect to viewing angles from the other color light-emitting materials. In detail, the short-wavelength red (R) light-emitting material used as a dopant shows a significantly high color change of about 0.120 at a left/right viewing angle of 60°.
Meanwhile, the green (G), blue (B), and white (W) light-emitting materials show color changes in a range from about 0.020 to about 0.040.
Next, in the case of the red (R) light-emitting material, changes in luminance with respect to viewing angles of white color are described with reference to FIG. 3C.
First, in a color coordinator, white shows a good change in luminance with respect to viewing angles as it moves in a left and down direction.
At this time, a change in luminance of white according to viewing angles of a light-emitting material including a short-wavelength dopant moves in a right and up direction, and then moves in a left and up direction while making a curve. In other words, a light-emitting material including a short-wavelength dopant shows a poor change in luminance of white according to viewing angles.
As described above, a red light-emitting diode RLE uses a short-wavelength dopant in order to increase luminous efficiency, however, the short-wavelength dopant causes problems of deterioration of viewing angle properties, deterioration of color change properties, and a sharp change in luminance.
Meanwhile, referring again to FIG. 2, the organic light-emitting layer is formed by a thermal deposition method of heating a source material in a chamber, and depositing the source material on a target.
FIG. 4 shows a deposition apparatus that has been used in a conventional thermal deposition method. Referring to FIG. 4, a source is disposed on the bottom of the deposition apparatus, and a substrate 20 is positioned above the source 10 while spaced by a first distance d1 from the source 10. The substrate 20 is rotated in the state that the locations of the source 10 and the substrate 20 are fixed, and when the source is heated, a source material is deposited on the substrate 20.
In order to deposit the source material on only a part of the substrate 20, a mask 30 having a plurality of openings 32 may be used.
According to the thermal deposition method described above, the first distance d1 between the substrate 20 and the source 10 needs to be far enough to deposit the source material on the entire area of the substrate 20. However, this became a factor of increasing the size of vacuum thermal evaporation equipment. Furthermore, due to the long distance between the substrate 20 and the source 10, a large amount of source material remains on the side walls of the vacuum thermal evaporation equipment, which causes a waste of substance.